Histone phosphorylation plays a direct role in mitosis, cell death, DNA damage repair, DNA replication and DNA recombination. For example, phosphorylation of the N-terminus of histone H3 may promote chromatin condensation during mitosis. Histone phosphorylation mostly occurs on serine (S) and threonine (T) residues, but also occurs on tyrosine (Y) residues. Histone phosphorylation and histone dephosphorylation are in dynamic equilibrium, regulated by protein kinases (PKs) and protein phosphatases (PPs). PK catalyzes the binding of the phosphate group to the amino acid residue at the tail of the histone, which is involved in the transcriptional activation of the gene. PP, on the other hand, removes the phosphate group from the amino acid residue and suppresses the initiation and expression of gene transcription.